Solid state and solution conformation of 6-{4-[N-tert-butoxycarbonyl-N-(N '-ethyl)propanamide]imidazolyl}-6-deoxycyclomaltoheptaose: Evidence of self-inclusion of the Boc group within the beta-cyclodextrin cavity

A new modified beta-cyclodextrin (beta-CD) derivative 1 that was functional ized in position 6 with Boc-Carcinine was synthesised and its crystal struc ture was determined. The structure reveals a "sleeping swan"-like shape, th e covalently bonded Boc-Carcinine moiety forming a folded structure with th e Boc group inserted within the hydrophobic cavity of the beta-cyclodextrin . The conformation of the Carcinine moiety is determined by the inclusion o f the Boc group and is further stabilised by three intramolecular hydrogen bonds, two between the amide N1-H group, the carbonyl C'1=O1 group and a pr imary hydroxylic group of the glucose unit 5, one between the carbonyl C'0= O0 group and the primary hydroxylic group of the glucose unit 2. The beta-C D macrocycle differs only slightly from unmodified beta-CDs, maintaining an approximate sevenfold symmetry. The solution structure of the new beta-CD derivative was investigated by NMR spectroscopy and circular dichroism (c.d .) spectroscopy. In addition to a complete (H-1 and C-13) assignment of the pendant Boc-Carcinine group, the NMR study allowed the assignment of all t he proton resonances associated with the beta-CD macrocycle. Furthermore, N MR and c.d, results indicated that the self-inclusion of the Boc group with in the beta-CD cavity is retained in aqueous solution. In order to estimate the strength of this self-inclusion complex a series of competition experi ments with the external guest 1-adamantanol was carried out using c.d, spec troscopy.